rmtsys: Don't overflow pathname buffer

When we're constructing a homedirectory path to look for the
.AFSSERVER file in, we copy the HOME environment variable into a
static buffer, with a risk of overflowing that buffer.

Instead of using a static buffer, just allocate one with asprintf.

Caught by coverity (#985910)

Change-Id: I2daa5613609f2c09712b12a7ce7e59b1c0028ef2
Reviewed-on: http://gerrit.openafs.org/9392
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>